FIG. 1 shows schematically a conventional industrial gas turbine engine 1 which comprises, in flow series through the engine, a compressor section 2, a combustor 3, and a turbine section 4. The engine works in a conventional manner so that air entering an intake 5 of the engine forms a flow into the compressor section, which compresses the flow before delivering the compressed air into the combustor where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through and thereby drive the turbine section before being exhausted through an exhaust 6. The turbine section is mounted on a shaft or coaxial shafts 7 with the compressor section and a generator 8. Power extracted from the working gas by the turbine section drives the compressor section and the generator.
Steam injection is employed on gas turbine industrial engines to reduce NOx emissions and boost power. The specific fuel consumption of the engine can also be improved as the steam is usually raised using engine exhaust heat, e.g. in a heat recovery steam generator (HRSG). For example, as described in EP A 1905964, steam may be injected into the vicinity of the outlet of the compressor section. It may also be injected into the turbine section where it beneficially cools the turbine and can provide an additional power boost. Steam is a much more effective cooling medium than compressor delivery air, due to its higher specific heat (approximately double that of compressor delivery air), higher conductivity, and lower temperature. Injecting steam into the engine is known as an “advanced cycle”.
In some gas turbine systems, steam is generated in a once through steam generator (OTSG) and used to power a steam turbine in addition to providing the steam for steam injection. The use of both a gas and steam turbine in this manner is known as a “combined cycle”. An OTSG does not have the thick walls of the drums of conventional HRSGs, which generally allows start-up times to be short. The short start-up time is achieved, in part, by the ability of the OTSG to run dry i.e. without any water flowing through the tubes on the secondary side of the heat exchanger.
A basic engine, i.e. without steam injection and without combined cycle, is generally able respond rapidly to load changes, and can provide a rapid start-up. For example, a 10 minute start-up period is an industry standard for engines in the 40-100 MW class (which may be aero-derivative engines). However, such an engine does not provide the operational benefits, such as improved efficiency and reduced emissions, that can be obtained through steam injection and combined cycle.